Thermoplastic film recording media



United States Patent 3,415,681 THERMOPLASTIC FILM RECORDING MEDIA James F. Burgess, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Dec. 14, 1964, Ser. No. 418,339 3 Claims. (Cl. 117-217) ABSTRACT OF THE DISCLOSURE This application discloses a deformation type thermoplastic recording medium in which a surface layer of material in excess of A. in thickness is applied to the surface of the medium for the purpose of improving the readout characteristics of a recording made on the material. The material is vapor-deposited in the preferred form of the invention, in an agglomerated or islandic structure and increases the voltage of the second crossover of the secondary emission characteristic or, in the case of metal coatings, provides a specular reflective surface for reflective optical reproduction on the stored information. The materials include metals and inorganic insulators.

This invention relates to improved media for deformation recording of information upon thermoplastic polymeric surfaces and to the subsequent retrieval of such information particularly by spectial reflective readout.

More particularly, the invention relates to an improved recording media incorporating a layer of thermoplastic polymeric material which is adaptable to be electrostatically charged in accordance with a predetermined pattern corresponding to a desired image. Upon heating, the image pattern is reproduced or developed as a pattern of ripples, grooves, or depressions in the surface of the thermoplastic layer by the application of heat, which pattern may be preserved or frozen in place by then cooling the thermoplastic layer. The information or other intelligence so-recorded may then be retrieved at will from the ripple pattern frozen on the surface of the thermoplastic layer. The improvement of this invention involves the provision of a surface modification of the thermoplastic layer which enhances the readout or retrieval of the recorded information.

An electron beam recording device which employs a solid recording medium having a thermoplastic surface layer on which television pictures and other forms of data can be permanently recorded has been described in United States Patent 3,113,179, issued on Dec. 3, 1963 in the name of W. E. Glenn, Jr., entitled Method and Apparatus for Recording, assigned to the assignee of the present invention. This technique has been applied to the storage of digital and analog data in forms other than a video picture. The storage of information in the form of .a heatdeveloped ripple pattern on the surface of a thermoplastic medium may be accomplished in other Ways. For example, the use of photoconductors in conjunction with electromagnetic radiation is disclosed in the United States patent application of J. Gaynor, Ser. No. 79,260, filed Dec. 29, 1960, now Patent No. 3,291,601, granted Dec. 13, 1966, and entitled Information Storage on Deformable Medium, and assigned to the assignee of the present invention. The retrieval of the information, image, or data so recorded may be accomplished in a variety of ways. For example, optical means, both transmissive and reflective, may be employed. A direct printing by applying ink or the like to the relatively raised portions of the ripple pattern followed by transferral of the ink to a sheet of paper may be used. The application of electrostatic powders to the surface of the recording media, taking advan- 3,415,681 Patented Dec. 10, 1968 tage of residual electrostatic charges in the form of the original charge pattern, will result in the powder clinging to either areas retaining a charge or to areas which bear no charge, depending upon the electrostatic characteristic of the powder, will produce either a positive or negative image, which may be directly utilized or, by known charge transfer techniques, may be transferred to another dielectric surface, such as paper. Yet, another technique which may be employed utilizes electron beam scanning of the information-bearing surface to produce secondary electron emission which may be detected and measured to retrieve the data. This so-called flying spot technique and apparatus for accomplishing it is more fully described in the copending United States patent application Ser. No. 140,849, filed Sept. 26, 1961 in the names of J. E. Wolfe and R. G. Reeves, now Patent No. 3,247,493, granted April 19, 1966, entitled Electron Beam Readout of Thermoplastic Recording, and assigned to the assignee of the present invention.

In general, this latter method of data or intelligence retrieval involves scanning the surface of the thermoplastic layer having the information deformations with a beam of primary electrons, collecting the secondary electrons which are emitted from the surface of the recording medium as a result of impingement thereon of the primary electron beam, and deriving a measurement of the number of secondary electrons collected as a measure of intelligence recorded by the deformations on the surface of the medium. In practicing this technique, a recording playback device is provided which comprises an electron gun assembly having a deflection means capable of deflecting the electron beam of the gun over a wide deflection angle. The electron gun assembly is supported in a vacuum tight housing along with the solid thermoplastic recording medium to be read out with the gun being positioned in a confronting relation with respect to the deformation-bearing surface of the recording medium. A collecting means is positioned within the housing in confronting relation with respect to the recording medium and is disposed so that it does not interfere with the primary electron beam. The collecting means serves to collect secondary electrons emitted from the surface of the recording medium as a result of the impingement thereon of the primary electron beam. The secondary electron current produced in the collecting means then provides a measure of the intelligence recorded on the recording medium.

Certain problems arise during the secondary electron emission readout procedure. It is known that the higher the velocity of the primary electrons, the easier it is to focus the electron beam and the smaller the size of the spot, resulting in greater resolution. The velocity of these electrons is expressed in volts. As the velocity of the primary electron beam is increased, the number of secondary electrons emitted from the thermoplastic recording surface increases in a curvilinear relationship until at a first voltage, the ratio of the number of incident or primary electrons to the number of emitted or secondary electrons is 1:1. This has been referred to as the first cross-over point. As the velocity of the primary electrons is further increased, the ratio of emitted electrons to incident electrons increases to a peak and then decreases to a point at which the ratio is again 1:1. This has been referred to as the second cross-over point. Further increase in velocity of the primary electron beam results in progressively 'fewer numbers of secondary electrons. When the velocity of the primary electrons is between the first and second cross-over points, a positive charge is produced on the thermoplastic recording member and when the velocity of the primary electrons exceeds that of the second cross-over point, a negative charge is produced on the thermoplastic recording member. The presence of either a positive or negative charge on the recording surface is undesirable since it affects the focus of the beam and the location of the spot and also deflects the paths of secondary electrons. Therefore, in order to operate such apparatus at the greatest efliciency, the velocity of the primary electrons should be maintained at a voltage substantially identical to the second cross-over point of the target material. The second cross-over point voltage for thermoplastic polymers suitable for recording members of the type hereinbefore described is of the order of 1000 to 1500 volts. It would be desirable to operate the readout apparatus at primary electron velocities of about 5000 volts or greater. It is known that by lowering the angle of incidence of the primary electron beam upon the recording surface from 90 to about 45, the effective velocity of the second cross-over point may be substantially increased, but the increased voltage is still short of the desired voltage range.

The advantages of operating at these higher voltages are: better focusing resulting in a smaller readout spot size which gives better resolution and therefore enables the use of a greater storage density in the recording member; and, faster readout speed. Claims directed to improved media for deformation recording having a surface layer overlying the deformable layer which has low lateral conductivity and which provides a secondary emission characteristic in which the second crossover is in excess of 2000 volts when scanned by an electron beam at 90 are presented in my divisional application Ser. No. 721,217, filed Apr. 15, 1968, and assigned to the assignee of this invention.

Under present practices, when optical reflective readout techniques are used, a highly reflective or specular layer is provided on a base and a recording layer of substantially transparent deformable thermoplastic polymer is superposed thereon. After the information to be recorded is frozen in the recording layer in a ripple pattern, reflective readout is achieved by projecting a beam of light through the recording layer and detecting variations in the light reflected from the specular layer. The readout beam must travel through the recording layer twice which reduces the attainable resolution, results in a low signal-to-noise ratio and is adversely affected by any decrease in the transparency of the recording layer which may occur through the thermal cycling necessary for repeated erasure and reuse of the recording medium. It is, therefore, desirable to eliminate the disadvantages resulting from the light beam having to pass through the transparent layer twice to produce a useful signal.

It is a primary object of this invention to provide an improved solid heat-deformable polymeric recording medium having a recording surface which enables a more efficient retrieval of information stored thereupon.

It is a further object of this invention to provide an improved solid heat-deformable polymeric recording medium having a recording surface which enables a more efficient retrieval of information stored thereupon by an optically reflective readout technique.

Other objects, features, and attendant advantages of this invention will become apparent from the detailed disclosure which follows.

Briefly stated, and in accordance with one embodiment of the invention, recording media comprising a thermoplastic solid polymer recording body having a recording surface is provided with a thin continuous film of an electrically nonconductive, or at best only semi-conductive, material which is effective to raise the voltage of the second cross-over point of the secondary electron emission of the thermoplastic by at least fifty percent without substantially affecting the ability of the thermoplastic recording layer to receive and retain an electrostatic charge and to thermally form a ripple pattern in accordance with the charge pattern. In addition, in the event a reflective or transmissive optical readout system is to be used, these coated recording members provide higher spectral reflectivity and useful transmissivities.

Similar surface films have been found to enhance the deformation sensitivity of the thermoplastic recording layer as disclosed in the copending United States applica tion Ser. No. 418,133, filed concurrently herewith in the name of K. W. Laendle, entitled Thermoplastic Film Composition, and assigned to the assignee of the present invention.

When previous attempts have been made to modify the recording surface of these thermoplastic recording media, it has been found that the ability of the surface to receive and retain an electrostatic charge has been drastically reduced or eliminated. Further, if the recording member has been chargeable, its ability to respond to heat to form a ripple pattern has been severely reduced.

It has been discovered, however, that by utilizing materials which may be vapor deposited upon the recording surface in the form of minute islands rather than as a continuous film, fairly thick films may be deposited upon the recording surface without exhibiting electrical conductivity, even though the materials per se may be electrical conductors. This phenomenon is thought to occur because some materials tend to deposit from the vapor state upon earlier deposited particles of the material or migrate to them rather than directly deposit on the substrate and, hence, tend to agglomerate to form minute islands of growing thickness which are probably physically separated from neighboring deposition sites. Films of vapor deposited metals such as tin, germanium, indium, and silver, for example, behave in this manner. Many other metals can be deposited in this manner as an agglomerated structure by using low evaporation rates or by heating the substrate. Volatilizable nonmetallic materials such as, for example, zinc sulfide, may also be so deposited.

Example 1 A thermoplastic recording layer 30 microns thick was prepared by casting a solution of a polystyrene having an average molecular weight of 20,000 dissolved in a mixed solvent consisting of benzene, toluene, and xylene on a 2 in. x 2 in. glass slide and evaporating off the solvent. The reflectivity of the polystyrene recording surface was measured over the visible spectrum and found to be approximately 10 percent. The recording surface was then exposed to tin vapor in a chamber which was evacuated to a pressure of about 10* mm. Hg for 10 seconds, resulting in the deposition thereon of a tin film about 100 A. thick. The reflectivity of the coated surface was measured and found to be specular. The reflectivity at 400 millimicrons wavelength was about 64 percent, at 500 millimicrons about 68 percent, at 600 millimicrons about 72 percent, and at 700 millimicrons about 72 percent. The resistance as measured on the 1 megohm scale of an ohmmeter was infinite. The film was an electrically discontinuous layer with high spectral reflectivity and the underlying recording layer was capable of accepting an electrostatic charge and of being thermally deformed in aceordance with the charge. The transmissivity of this example was measured across the visible spectrum and found to be about 8 percent at 400 millimicrons and to decrease in an almost linear fashion to about 4.5 percent at 700 millimicrons.

Example 2 Polystyrene coated slides were prepared in the same manner as set forth in Example 1 and similarly provided with discontinuous surface coatings about 100 A. thick of tin, zinc sulfide, and silver. The secondary electron emissivity of these slides as well as a slide having an uncoated polystyrene film were measured to determine their respective secondary electronic cross-over points using a incidence angle beam, e.g., perpendicular to the surface.

Material: Secondary cross-over, volts Uncoated polystyrene 1300 Tin 3200 Zinc sulfide 5100 Silver 2260 Since it is known that decreasing the angle of incidence of the electron beam from 90 to between 45 to 60, the second cross-over voltages of such materials increase about 100 percent, these materials are, therefore, operable in voltage ranges between about 5,000 to 10,000 volts. As before, these coatings did not interfere with the ability of the polymeric material to accept and hold an electrostatic charge nor to thermally deform in response thereto. They were charged with 1,000 volts DC and upon heating, a deformation pattern developed which was capabl of readout.

When known optical readout techniques are to be employed, the thickness of the specular films may be advantageously reduced so that the amount of transmitted light may be increased, permitting both reflective and transmissive readout to be accomplished. In this manner, if done simultaneously, the information obtained by one method may be simultaneously checked against the information obtained by the other method. The upper limit for the thickness of such specular films is about 150 A., above which the transmissivity decreases to a point where its usefulness ceases. Film thicknesses of the order of A. to 20 A. may be used, but when the thickness approaches 300 A., the ability of the underlying thermoplastic layer to deform is impaired.

One of the major advantages of the highly reflective surface for optical readout is that the top surface is used to reflect the signal. This will permit higher efiiciency and resolution since the signal will not be affected by particles or other light scattering imperfections in the thermoplastic film or base.

In addition to the materials specifically disclosed by the foregoing examples, other metals and compounds may be advantageously employed to form the deposited surface film. For example, gold behaves very similarly to tin in that it tends to form agglomerated deposits in thin films rather than a continuous film and has a secondary crossover point in excess of 2,000 volts. Chemical compounds which have desirable properties include the alkaline metal halides such as KBr, KCl, KI, NaBr, and NaCl, for example, and oxides such as BeO and MgO, for example. It will also be appreciated that only when the surface film material is electrically conductive in itself is it necessary that the film be composed of discontinuous islands. Where the film is formed of an essentially nonconductive material, substantially continuous films may be used.

In the preceding disclosure, the deposited surface films have been specifically set forth as being formed by evaporating the materials involved and condensing such vapors upon the surface of the thermoplastic substrate. Other methods of producing equivalent films, such as by vapor phase reactions, sputtering, electroless plating, fluid bed plating techniques, and others, may be employed if desired.

While a number of specific materials have been recited in the foregoing description as exemplary of the invention, it is not intended to limit the invention in any manner except as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An information storage medium comprising a layer of a thermoplastic material supported on one surface, said thermoplastic layer, when heated, being deformable under the influence of electrostatic forces produced therein in a pattern corresponding to th information to be stored and a metallic coating on the opposite surface of said layer having a thickness greater than 10 A. and an agglomerated structure, said metallic surface having very low lateral electrical conductivity and being heat-deformable along with the thermoplastic layer upon the application of heat to provide a specular reflective surface for refiective optical reproduction of the stored information.

2. The information storage medium of claim 1 in which the constituents of said metallic coating are selected from the group consisting essentially of indium and tin.

3. The information storage medium of claim 1 in which the thickness of said metallic coating is vapor-deposited to a thickness in the order of A.

References Cited UNITED STATES PATENTS 3,247,493 4/1966 Wolfe et al 340-173 3,268,361 8/1966 Gaynor 117217 X 3,291,601 12/1966 Gaynor 9611 3,317,315 5/1967 Nicoll et a1 117217 X ALFRED L. LEAVITT, Primary Examiner.

C. K. WEIFFENBACH, Assistant Examiner.

US. Cl. X.R. l17--107, 211 

